[PATCH 2/3] drm: Make drm_mode_vrefresh() a bit more accurate

[Daniel Vetter]

On Tue, Mar 13, 2018 at 05:07:58PM +0200, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Do the refresh rate calculation with a single division. This gives
> us slightly more accurate results, especially for interlaced since
> we don't just double the final truncated result.
> 
> We do lose one bit compared to the old way, so with an interlaced
> mode the new code can only handle ~2GHz instead of the ~4GHz the
> old code handeled.
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

I kinda want special integers here that Oops on overflow, I thought
they're coming. That aside:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> ---
>  drivers/gpu/drm/drm_modes.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> index 4157250140b0..f6b7c0e36a1a 100644
> --- a/drivers/gpu/drm/drm_modes.c
> +++ b/drivers/gpu/drm/drm_modes.c
> @@ -773,24 +773,23 @@ EXPORT_SYMBOL(drm_mode_hsync);
>  int drm_mode_vrefresh(const struct drm_display_mode *mode)
>  {
>  	int refresh = 0;
> -	unsigned int calc_val;
>  
>  	if (mode->vrefresh > 0)
>  		refresh = mode->vrefresh;
>  	else if (mode->htotal > 0 && mode->vtotal > 0) {
> -		int vtotal;
> -		vtotal = mode->vtotal;
> -		/* work out vrefresh the value will be x1000 */
> -		calc_val = (mode->clock * 1000);
> -		calc_val /= mode->htotal;
> -		refresh = (calc_val + vtotal / 2) / vtotal;
> +		unsigned int num, den;
> +
> +		num = mode->clock * 1000;
> +		den = mode->htotal * mode->vtotal;
>  
>  		if (mode->flags & DRM_MODE_FLAG_INTERLACE)
> -			refresh *= 2;
> +			num *= 2;
>  		if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
> -			refresh /= 2;
> +			den *= 2;
>  		if (mode->vscan > 1)
> -			refresh /= mode->vscan;
> +			den *= mode->vscan;
> +
> +		refresh = DIV_ROUND_CLOSEST(num, den);
>  	}
>  	return refresh;
>  }
> -- 
> 2.16.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch